The present invention relates to medical diagnostic ultrasonic imaging systems, and in particular to improved signal paths that improve resolution and reduce speckle variance in such imaging systems.
Speckle is generated by interference from waves emanating from multiple scatterers within the sample volume (resolution cell) of an ultrasound imaging system. For log-compressed images, speckle acts as additive noise that interferes with the detectability of small and/or subtle lesions that only have a small echogenicity difference as compared to adjacent tissues. For this reason, speckle variance reduction is critical to enhancing detectability of tissue echogenicity differences.
Several speckle reduction techniques are widely used in the medical ultrasound field. Most employ some form of spatial, temporal, or frequency compounding. For example, post-detection video filtering, persistence filtering of spatially uncorrelated images, and similar methods are used. These methods cause a loss of spatial or temporal resolution, or both.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the ultrasonic imaging systems described below include a signal path that reduces speckle while preserving or even improving detail resolution, depending on the amount of speckle reduction desired. The signal path includes a whitening filter such as a two-dimensional whitening filter positioned upstream of the detector, and a low pass filter such as a two-dimensional low pass filter positioned downstream of the detector. The disclosed signal path thus provides pre-detection spatial frequency whitening followed by post-detection spatial smoothing. In some of the embodiments described below, the amplitude of the whitened ultrasonic receive signal is adjusted to suppress side lobes that may be associated with spectrum whitening.